The present invention concerns a method and an apparatus for defrosting frozen logs before debarking. The large production volume of the present wood debarking lines also with frozen logs requires effective defrosting prior to the debarking in a drum. The defrosting conveyor used for this purpose is an expensive apparatus taking a lot of space, its size being dependent on the required defrosting time.
With the defrosting conveyors of prior art, the defrosting is usually implemented so that water of a certain temperature is sprayed on the stream of logs moving on the conveyor. The defrosting is intensified by decreasing the speed of the conveyor, by properly filling up the conveyor, or by increasing the length of transport during the defrosting. Usually, the amount of energy available for defrosting is stable during the whole defrosting process.
According to an older defrosting process, the debarking drum comprised a starting sector with no bark openings, partly filled with water, into which warm water was pumped in the winter time. Due to water pollution problems, these so called wet-debarking drums have today been abandoned. There are presently also various ways of supplying energy, depending on the mill. In new mills, the defrosting energy is delivered by means of a large amount of water of a relatively low temperature, by means of a small amount of water with high temperature, by means of superheated, pressurised water, or by steam.
In the transverse conveyor system of Swedish origin, the defrosting is implemented in two phases, wherein the log bundles initially are defrosted as a large pile, and after the pile has been lowered, an effective after-defrosting takes place.
Experience shows, that a defrosted layer of about 10 mm from the bark surface of the log is required to achieve a good debarking result. It is, however, difficult to provide defrosting evenly for the whole log layer to be transported. Especially large logs are only partly defrosted, and with temperatures of several degrees below zero, the logs freeze again, because the cold heartwood forms a prominent part of the log. There are also problems with the logs on the bottom of the log pile, which are defrosted only by the colder water.
The process of defrosting logs, including heat transmission and temperatures of different wood layers, has been described in the following publications, among others:
Thermal Conductive properties of wood, Green or Dry, USDA Forest Service General Technical Report FPL-9
Heating Frozen and Nonfrozen 1977 Veneer Logs, Forest Products Journal 1972 Vol 22. No. 10
Oppvarming og rengjxc3x6ring av skurtxc3x6mmer fxc3x6r barkning, Norsk Treteknisk Institut Rapport Juni 1979
Upptining av frusen ved, Svenska Forskningslaboratonet, report 549, 11.07.1966 and report 575, 16.11.1966.
The log defrosting method according to the present invention is based on zone-specifically providing heat to the logs in transfer motion on a conveyor, according to their heat receiving capacity immediately from the moment of starting of the defrosting, and at the end of the defrosting process, the amount of heat provided per time unit (length of path) is only a fraction of the amount of heat in the first defrosting phase. Preferably, the heat amount ratio is about 5:1. The defrosting conveyor can be divided into 3 to 10 defrosting zones with a length of, for example, 2-10 meters each. Application of the present invention is particularly suitable for the energy delivery methods mentioned on page 1. By means of the invention, the defrosting energy can be more efficiently utilised, and at the same time water pollution is decreased.
The above mentioned invention is based on the following starting values received from the reference literature:
Heat transmission to the surface of the logs is restricted by the surface resistance.
Thermal conduction from the surface of the logs through the bark and into the wood, causing melting of ice. Thermal conductivity of woodxe2x80x941 KJ/m2hxc2x0 C. (Moisture 50%, old value 0.24 kcal/m2hxc2x0 C.).
Based on the foregoing, the thermal conductivity to different depths from the surface of the log can be calculated.
Based on the heat transmission value and the thermal conductivity to the layer to be defrosted, the surface temperature of the log can be calculated, which gives the probable defrosting depth. At the end of the defrosting process, when the defrosted layer is 10 mm thick, the wood surface receives only about 5% of the heat amount of the starting phase of the defrosting process.
According to the new, intensified defrosting method, heat is brought zone-specifically, according to their heat receiving capacity, to the logs to be defrosted while the logs being transported on a conveyor and the total heat amount is increased in a restricted manner, only as a result of better defrosting.